The present invention relates to an electrophotographic apparatus comprising an improved bias voltage source for a magnetic brush development means.
Although a rotary drum having a photoconductive layer formed on the periphery thereof is in widespread use in electrostatic copying machines, the entire drum must be replaced when the photoconductive layer is worn out through prolonged use. This involves considerable maintenance costs since the drum is expensive to manufacture and the replacement operation can be quite time consuming.
For this reason, a photoconductive sheet has been developed which may be wrapped around and attached to a metal drum core or carrier which serves the same function as a photoconductive layer which is vacuum deposited or otherwise formed on the drum. Considerable economies are effected by this arrangement since the sheet is extremely inexpensive to replace compared to an entire drum and the sheet replacement operation may be performed quickly. Generally the sheet does not cover the entire periphery of the drum, and a portion of the metal drum core remains exposed.
It is also currently popular in the electrophotographic art to utilize a magnetic brush as a development means. More specifically, a photoconductive drum is charged and subsequently radiated with a light image of an original document to form an electrostatic image. The magnetic brush applies a toner developing substance to the drum which adheres to high potential areas of the electrostatic image to provide a visible toner image. This toner image is transferred to a copy sheet and fixed thereto to provide a permanent copy of the original document.
The magnetic brush typically comprises a magnetic core and an electrically conductive sleeve surrounding the core. Either or both of the core and sleeve are rotated and the sleeve is partially immersed in the toner substance so that the toner substance adheres to the sleeve and rotates in brushing engagement with the drum to develop the image thereon. An electrically insulative layer is formed on the sleeve to improve image resolution and prevent electrical conduction through any pinholes in the photoconductive layer of the drum. A bias voltage is applied to the sleeve of a magnitude sufficient to prevent adherence of toner to white or background areas of the electrostatic image.
When a drum comprising a metal core and a photoconductive sheet wrapped therearound is combined with the magnetic brush developing means described above, a problem occurs in that the background area near the leading edge of the electrostatic image attracts toner substance so that the background area of the resulting copy will be gray for a distance of several centimeters from the edge corresponding to the leading edge of the image. It is this problem which is overcome by the present invention.